To begin to understand the functional organization of the eukaryotic 40S ribosomal subunit with respect to mRNA and tRNA binding, initiation factor association, and subunit joining, the higher order structure of its 18S rRNA component must be determined. Secondary structure models for eukaryotic 18S rRNA are at present crude, and are based almost entirely on extrapolations from the proposed prokaryotic models by searching for compensating base changes in the yeart and frog 18S rRNA sequences. We are deriving a secondary structure model for rabbit 18S ribosomal RNA within the 40S subunit, compatible with data obtained using nucleases, chemicals, and psoralen as structure probles on both native 18S ribosomal RNA and purified 40S subunits. Rabbit 40S subunits can be purified from reticulocytes in high enough quantities necessary to carry out these studies. We are also mapping the topography of mammalian 18S ribosomal RNA within a functionally engaged ribosome, using both chemicals and psoralen as structure probes. Computer-aided predictions based on both thermodynamic energy minimizations and structure data are likely to generate convincing models for mammalian 18S rRNA in both the active and inactive states. Since eukaryotic 18S rRNA likely associated with other RNA components within the polyribosome complex, we want to precisely map these rRNA:RNA interactions by photo-induced psoralen crosslinking. The establishment of a fundamental conformation for 18S rRNA within the 40S subunit is essential in defining the multiple events occurring during translational control of gene expression in mammalian cells.